A theoretical and simulated calculation of the processes proceeding in a gain generator of a combustiondriven cw DF chemical laser with preheated primary dilution He was reported. Investigation results demonstrate that for DF lasers with fuel systems of (H₂+NF₃+He)+D₂ and (C₂H₄+NF₃+He)+D₂, when the primary dilution He is preheated to (1100 K,1300 K), the available specific output power is improved by (21%, 24%) and (46%, 56%) respectively; and at the same time, the fuel consumption in the combustor is reduced by (255%, 29%) and (32%, 36%) respectively. In practical application, the primary dilution He can be heated to 800℃(1100 K) successfully. Therefore, preheating the primary dilution He is a feasible and effective method to improve the laser output characteristics, which can facilitate the development of more compact chemical lasers with higher efficiency.